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Rock types have always gotten the short end of the stick in pokemon games. In gen I, they were THE defensive pokemon. Nobody could stand in their way...except fighting, water, grass, and ground types...all of whom were fairly common. They couldn't take psychic pokemon like Alakazam since he just dominated everything, and water and grass types were common all around too. Oh yeah, and most of them had a double weakness to something like Grass or water, making them an even easier kill.

In Gen II/III, they had their thunder stolen by steel types, who not only resisted much more than them, but were even super effective against them, adding insult to injury. It wasn't until Gen IV that rock got a powerful offensive move in stone edge, but even that is unreliable. Rock types got stealth rock in gen Iv, but so did just about everything else. Once again, rock types are just that other type nobody uses except to use rock slide or stone edge.

Steel and ground types have gotten around fine, but rock types pretty much got screwed all the way, so the special defense boost is a way of helping them. The last thing we need is steel types with boosted up special defense. Imagine how painful it would be to take out something like Scizor, Foretress, or Ferrothorn!

Well, there is a reason. I will give you the definition of Sand first:

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles. The composition of sand is highly variable, depending on the local rock sources and conditions. When combined by Wind, it forms a Sandstorm which is basically Particles of Rock.

The reason it only boosts Rock type stats is because technically the rocks from the sandstorm will attach to the Pokemon thus building its own defence by Filling its weak spots. Steel types and Ground types don't need rocks to boost Defence since they are not made of Rock. It raises its Sp Defence and Water types Get benefits from the rain like abilities Rain Dish and Swift Swim and it boosts the power of Water type moves.